common use is in estimating the elevation on a datum bed below the total depth 

 of a well that has penetrated a higher known stratigraphic horizon. But there 

 are many other practical uses, some of which are described below. 



Figure 24-15 is a subsurface structural map on the top of the Pennsylvanian. 

 In the northwest quadrant is an anticline with more than 100 feet of closure. The 

 southward-plunging anticline on the east side is open on the north end. These 

 structural contours are shown as dotted lines in Figure 24-16. The thickness of 

 the Pennsylvanian is shown by solid isopachous contours. Obviously, the 900 

 feet of convergence over the map area will have a profound effect on the form 

 of the structure at the base of the Pennsylvanian or at the top of the Mississip- 

 pian. The procedure for reducing the Pennsylvanian structure to the Mississip- 

 pian is as follows. 



Point a in Figure 24-16 is the intersection of the 1500-foot structure con- 

 tour and the 500-foot isopach contour. At this point the top of the Mississippian 

 is 500 feet below the Pennsylvanian datum at an elevation of 1000 feet. At 

 point b the Mississippian is 700 feet below the structure datum, and the elevation 

 is, therefore, 800 feet. All intersections are reduced to Mississippian elevations 

 in this manner, and these values are then contoured, as shown in the Mississip- 

 pian structure map in Figure 24-17. Now, if it is desired to determine the 

 structure on the top of the Devonian, the Mississippian isopach is superimposed 

 on the Mississippian structure, as in Figure 24-18, and the process just de- 



Boo / ^-N. \ \ ™? 



I t> 1 1 



Figure 24-17. Subsurface structural map on top of Mississippian. 



472 



